In general, hot strips are manufactured in the following manner: A slab is heated to a predetermined temperature in a heating furnace. The heated slab is rolled by using a roughing stand, whereby a rough bar having a predetermined thickness is obtained. The rough bar is rolled by using a continuous finishing stand constituted by a plurality of rolling stands, whereby a steel strip having a predetermined thickness is obtained. The steel strip is cooled by using a cooling device provided above a run-out table and subsequently is coiled by using a down coiler.
In this process, in the cooling device provided above the run-out table for continuously cooling the hot steel strip that has been subjected to hot rolling, a plurality of linear laminar flows of coolant are ejected from round-type laminar-flow nozzles onto roller-tables for conveying the steel strip over the width of the roller-tables, so as to perform upper-side cooling. On the other hand, lower-side cooling is generally performed by ejecting coolant from spray nozzles disposed between the roller-tables.
However, such a conventional cooling device, in which the round-type laminar nozzles used for upper-side cooling eject coolant in a free-fall-flow form, has problems including the following. Residual coolant on the steel strip may prevent coolant from reaching the steel strip, and thus producing variations in coolability in the cases of having and not having residual coolant on the steel strip. Moreover, coolant that has fallen onto the steel strip spreads in arbitrary directions, thereby producing variations in the cooling zone, leading to thermal instability in cooling. As a result of such variations in coolability, the quality of the steel strip tends to become nonuniform.
To obtain stable coolability by purging coolant on the steel strip (residual coolant), some methods have been proposed including the following: a method in which residual coolant is removed by obliquely ejecting fluid in a direction crossing the upper surface of the steel strip (see Patent Document 1, for example); and a method in which uniformity in the cooling zone is obtained by blocking residual coolant using constraining rolls, serving as purging rolls, for constraining the vertical movement of the steel strip (see Patent Document 2, for example).
Cited Patent Documents are listed below, including Patent Document 3, which will be cited in Best Modes for Carrying Out the Invention.                Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-141322        Patent Document 2: Japanese Unexamined Patent Application Publication No. 10-166023        Patent Document 3: Japanese Unexamined Patent Application Publication No. 2002-239623        